How to design a part for CNC milling?

Milling cost is created first in CAD and only then on the machine. The same part function designed in two different ways can differ in price and lead time by tens of percent. Design for machining, or DFM, is a set of simple geometry rules that shorten cycle time and reduce the risk of problems.

This post collects the most important rules for design engineers. If you are still deciding on the machining strategy, read it together with 3-axis vs 5-axis milling, because the number of axes also follows from the geometry of the part.

Round the internal corners

An end mill has a round cross-section, so an internal corner always keeps a radius. Designing sharp internal corners forces the machinist into longer machining with a thinner tool or into a different technology.

• Assume a corner radius at least equal to the radius of a typical end mill,
• a larger radius allows a thicker, stiffer tool and a shorter cycle time,
• sharp external corners are not a problem — this applies only to internal corners.

In practice, a corner radius slightly larger than the cutter radius gives the best compromise between function and machining cost.

Watch pocket proportions and depth

Deep, narrow pockets require long tools that vibrate and lose accuracy. The deeper a pocket is relative to its width, the harder and more expensive the machining.

General rule: without special tooling a sensible depth is about 3–4 cutter diameters. Deeper pockets are feasible, but they should be consciously justified by the function of the part.

Do not design walls that are too thin

Thin walls vibrate and deflect under tool pressure, which spoils accuracy and surface finish. The problem grows with the height of the wall.

For metals a minimum of about 0.8–1 mm is often assumed, but that is a starting point, not a universal rule. A tall, thin wall requires a higher threshold and sometimes additional technological supports. If a wall has to be thin, discuss it with the machining supplier before production.

Give the tool access

The cutter must reach every machined surface. Closed geometry, deep undercuts and "hidden" surfaces require extra setups, longer tools or 5-axis machining.

• Design so that the most important surfaces are accessible from one or two directions,
• avoid undercuts that require angle tools unless they are necessary,
• remember that every additional setup means cost and a potential datum error.

The number of setups translates directly into price — more on that in how much does a CNC part cost.

Tolerances and threads — only where needed

A tight tolerance across the whole part sounds safe, but it costs money. Every tightened tolerance means longer machining, extra inspection and sometimes a separate operation.

• Mark as critical only functional surfaces, fits and assembly dimensions,
• leave the remaining dimensions at general workshop tolerance,
• design threads in standard sizes for which standard tools exist,
• avoid very deep threads in hard materials unless they are necessary.

How to state these requirements sensibly on the documentation is described in how to prepare a drawing for a quote.

Decision framework: a quick DFM review

Before you send the design for a quote, check:

Summary

A good design for CNC milling is not a compromise on function but a set of conscious geometric decisions: corner radii, pocket proportions, wall thickness, tool access and tolerances limited to what really matters. Each of these decisions cuts cost and risk before the part reaches the machine.

Have a finished model and want it checked for machinability? Send the design to Nomatec — as part of CNC milling and CAD/CAM design we will point out what to simplify so the part is cheaper to produce without losing its function.